A direct injection internal combustion engine has been popular. The direct injection internal combustion engine, which is referred to as a DI engine hereinafter, has characters such as a high fuel economy, a low emission and a high power. The DI engine is operated in either a stratified combustion mode or a homogeneous combustion mode. The stratified combustion mode is carried out in a case of a low-load of the engine, and the homogeneous combustion mode is carried out in case of a middle and high-load of the engine.
In order to improve a combustion stability in the stratified combustion mode, it is proposed that a fuel injection timing and/or a pressure of injecting fuel, which is referred to as a fuel pressure hereinafter, is varied. JP-5-79370A shows that a fuel injection timing is varied according to an engine load in the stratified combustion mode. That is, the fuel injection timing is advanced to a primary stage of intake stroke when the engine load is lower than a predetermined value, and is retarded to a middle stage of intake stroke when the engine load is higher than the predetermined value.
Generally, since the fuel injection amount is increased in a high-load of the engine, a period for atomizing the injected fuel increases. In JP-5-79370A, since the fuel injection timing is retarded to the middle stage of the intake stroke in case of the high-load of the engine, a period from the fuel injection timing to an ignition timing is reduced. When a mount of injected fuel is relatively large and/or a velocity of intake air is relatively low, the ignition is carried out before the injected fuel is atomized enough, so that an emitted amount of hydrocarbon (HC) and a smoke can be increased. When the fuel injection timing is advanced to the primary stage of intake stroke in the case that the engine load is lower than the predetermined value, the period for atomizing the injected fuel is enough. However, a position of a piston is close to a top dead center (TDC) and a fuel injector, so that an amount of fuel adhering to the piston can be increased to deteriorate the emission and to generate a smoke of a spark plug. The fuel adhering to the piston is referred to as a piston-wet hereinafter.
JP-10-131786A shows that the fuel pressure is varied according to a speed of a vehicle in case of the homogeneous combustion mode. When the speed of the vehicle is low, the period from the fuel injection timing to the ignition timing is increased. Considering such a situation, the fuel pressure is decreased and the fuel injection period is increased to expedite a mixture of injected fuel and an intake fresh air. In the event of high speed of the vehicle, the fuel pressure is set to a high value to decrease the injection period.
However, since the amount of injected fuel is increased in the case of the high speed of the vehicle, a high fuel pressure causes an increase of the piston-wet, so that the emission can be deteriorated and the smoke of the spark plug can be generated.